The present invention relates to a fiber reinforced thermoplastic structure having high electromagnetic/radio frequency interference (EMI/RFI) shielding effectiveness, and more particularly relates to an EMI/RFI shielding structure, and process for making the same, that has a thermoplastic layer containing a network of conductive fibers held in a thermoplastic matrix.
Electronic equipment, particularly sensitive electronic equipment such as computers, business machines, communications equipment and the like are all susceptible to malfunction as a result of EMI/RFI. Furthermore, in addition to being sensitive to foreign EMI/RFI, many electronic devices generate EMI/RFI, and if not properly shielded, emit large quantities of EMI/RFI energy. During the early years of the electronic age, EMI/RFI shielding of electronic equipment was accomplished by conductive metallic housings. However, with the boom in the use of non-conductive plastic materials in the electronic industry, particularly as sturdy, lightweight housings, EMI/RFI has become a great problem.
Much research has been undertaken to provide plastic housings having EMI/RFI shielding effectiveness. Until recently, EMI/RFI shielding effectiveness in plastics was accomplished by conductive coatings, metallization, and plating of molded plastic parts. These methods, while effective, are costly and labor intensive in that they require substantial amounts of material and involve secondary operations in preparing the final product.
Recently, attempts have been made to prepare conductive plastics by incorporating in engineering thermoplastics certain conductive fillers. Specifically, these fillers include conductive powders, flakes and fibers. Generally, approximately 25-40% by weight conductive powder, 36-49% by weight conductive flake or 25-30% by weight conductive fiber must be incorporated into plastic materials in order to obtain the desired levels of EMI/RFI shielding. Parts made therefrom have generally been made by injection molding processes. (Materials Engineering, March, 1982, P. 37-43; Modern Plastics International, September 1982, P. 46-49).
Injection molding of structures containing conductive fibers has had a number of shortcomings. Namely, the extruder screw used to injection mold blends of thermoplastic resin and conductive fiber has had a tendency to tear the fibers, reducing the average lengths from an initial fiber length of about 1/4 inch to a final average length of about 1/32 inch in the molded structure. These shorter fibers are less efficient in forming a continuous conductive network and thus require relatively higher loadings to achieve the same shielding effectiveness as can be achieved by lower loadings of longer fibers. In other words, loadings of shorter fibers are less effective in EMI/RFI shielding than are similar weight percent loadings of longer fibers. Producing EMI/RFI shielding structures from conventional injection molding also typically requires that the fibers be dispersed throughout the structure often requiring large amounts of the relatively expensive conductive fibers.
Accordingly, it is one object of the present invention to provide a thermoplastic structure employing relatively longer conductive fibers than is found in injection molded structures.
It is another object of the present invention to provide a structure which concentrates the fibers into a relatively thin layer of the structure to reduce the total loading of conductive fibers while obtaining a desired level of EMI/RFI shielding.